This invention relates to methods and systems for determining a position of a transceiver unit, such as those provided on an airplane or a surface vehicle, utilizing two-way ranging through multiple satellites.
Current Automatic Dependent Surveillance (ADS) technology, such as Global Positioning System (GPS), Wide Area Augmentation System (WAAS) or GLONASS, provides positioning information utilizing satellite transmissions. For example, the GPS, developed and deployed by the U.S. Department of Defense, consists of 24 satellites orbiting the earth twice a day at an altitude of 12,000 miles, as well as five ground stations to monitor and manage the satellite constellation. Using atomic clocks and location data, GPS satellites transmit continuous time and position information 24 hours a day to a GPS receiver, which listens to three or more satellites at once to determine the user""s position. By measuring the time interval between the transmission and the reception of a satellite signal, the GPS receiver calculates the distance between the user and each satellite, and then uses the distance measurements of at least three satellites to arrive at a position.
Such systems, however, utilize one-way ranging in which an accurate, synchronized clock is required at each station. Any synchronization error or error regarding the location of one of the satellites results in an error in the determined position of the target vehicle. Thus, there is a need to provide very accurate position and velocity information with a high degree of integrity and reliability.
It is thus a general object of the present invention to provide a method and system for determining a position of an object, such as an airplane or a surface vehicle, utilizing two-way ranging in a polystatic satellite configuration to derive independent estimates of the transceiver""s state vectors including position and velocity.
In carrying out the above object and other objects, features, and advantages of the present invention, a method is provided for determining a position of the object. The method includes transmitting a first ranging signal from a first satellite at a first known location to the object as directed by a ground station, transmitting a second ranging signal from the object to the first satellite in response to the first ranging signal for receipt by the ground station, transmitting a third ranging signal from one of the first satellite and a second satellite at a second known location to the object as directed by the ground station, transmitting a fourth ranging signal from the object to the other one of the first satellite and the second satellite in response to the third ranging signal for receipt by the ground station, determining a first delay corresponding to a time difference between transmission of the first ranging signal and receipt of the second ranging signal, determining a second delay corresponding to a time difference between transmission of the third ranging signal and receipt of the fourth ranging signal, and determining the position of the object based on the first and second known locations of the first and second satellites, respectively, and the first and second delay.
In further carrying out the above object and other objects, features, and advantages of the present invention, a system is also provided for carrying out the steps of the above described method. The system includes a first satellite communication transceiver at a first known location for providing a bidirectional communication paths between the first satellite communication transceiver and the object wherein the first satellite communication transceiver transmits a first ranging signal to the object as directed by the ground station and wherein the object transmits a second ranging signal to the first satellite communication transceiver in response to the first ranging signal. The first satellite communication transceiver also provide a first unidirectional communication path between the first satellite communication transceiver and the object corresponding to the first satellite communication transceiver performing one of transmitting a third ranging signal to the object and receiving a fourth ranging signal from the object. The system further includes a second satellite communication transceiver at a second known location for providing a second unidirectional communication path between the second communication transceiver and the object corresponding to the second satellite communication transceiver performing one of receiving the fourth ranging signal from the object in response to the first satellite communication transceiver transmitting the third ranging signal to the object and transmitting the third ranging signal to the object wherein the object transmits the fourth ranging signal in response thereto to the first satellite communication transceiver. The system further includes a ground station for determining a first path length corresponding to a first time length of the bidirectional communication path, determining a second path length corresponding to a second time length of the first and second unidirectional communication paths, and determining the position of the object based on the first and second known locations and the first and second path lengths.
The above object and other objects, features and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.